Fluid-charged heating systems are well known and in widespread use. Such systems typically circulate a working fluid to which and from which heat is transferred through a closed system of ducts from a heat source to a heat sink and back to the heat source. During this circulation, the working fluid goes through a working cycle that involves the transfer of heat to and from the working fluid. For example, water can be heated by a heat source such as an oil-fired boiler in the basement of a building, and the heated water can then be circulated in a closed loop to radiators in other parts of the building. In systems of this type passive thermal circulation can be utilized to circulate the working fluid. The heated water and steam rises from the basement boiler to radiators in the upper stories of the building where the heat can be extracted into the living quarters (heat sink). Then the relatively high-density, cooled water flows by gravity back to the basement heat source for reheating. Such a system is called a passive heat transfer system because no pumps or external power source (other than the energy used to heat the working fluid at the heat source) is required to circulate the working fluid from the heat source to the heat sink and back to the heat source.
Passive, closed, fluid-charged solar collector systems similar to the above-mentioned oil-fired burner system have been devised. In such a system, a storage tank (heat sink) is mounted above the solar collector. Working fluid that is heated within the solar collector rises from the heat collector into the storage tank. Heat can be removed from the heated working fluid in the storage tank as needed, either through a heat exchanger or by directly replacing heated fluid with cold fluid. Working fluid from which the heat has been spent is thermally circulated by gravity flow back to the solar collector. Although such passive solar collector systems have the advantage of being mechanically simple, practical limitations on storage tank size because of structural strength requirements and aesthetic considerations severely limit their use. Therefore it is often necessary or desirable to locate the heat sink below the heat collector.
Active systems that include additional mechanical apparatus such as pumps have been used to transfer heated working fluid from a rooftop solar collector or other heat source to the lower stories of a building where the heat can be used or stored. For example, FIG. 1 shows the major components of a conventional heat transfer system 10 that actively circulates working fluid to a heat sink 12 that is located below a heat collector 14. The heat collector 14, which in this case is a rooftop solar collector, is in fluid communication through a collector drain duct 16 with the heat sink 12. The heat sink 12 is in fluid communication through a collector feed duct 18 with the heat collector 14. A check valve 20 is positioned in the collector feed duct 18 to prevent back flow from the heat collector 14 through the collector feed duct 18 to the heat sink 12. The check valve 20 prevents the thermal circulation (indicated by arrows 21) from reversing, which would passively remove heat from storage, when there is no solar energy input to the heat collector 14. An expansion tank 22 and an electrical pump 24 are also provided in the collector feed duct 18. The expansion tank 12 accommodates the volume change caused by thermal expansion of the working fluid. Temperature probes 26, 28 are provided in the heat collector 14 and in the heat sink 12. Wiring 30 connects the temperature probes 26, 28 to a control box 32 and electrically couples the pump 24 through the control box 32 with a power source 34. Whenever there is a temperature difference of sufficient magnitude between the working fluid contained in the heat collector 14 and the heat sink 12, the control box 32 activates the pump 24, and the working fluid is pumped through the system 10 at a constant rate.
Compared to conventional passive systems, conventional active systems characteristically exhibit a higher degree of complexity because of their dependence upon components such as pumps and temperature control elements and upon an external power source. Mechanical complexity, presence of moving parts, use of electric and electronic components all contribute adversely to system cost, reliability, and operating life. Dependence upon external power limits the use of active systems in underdeveloped regions. However, active systems do accomplish what conventional passive systems cannot do: the transfer of heat from a heat source to a heat sink that is positioned below the heat source.
In attempts to overcome at least some of the disadvantages of active systems, several passive heat transfer systems have been proposed wherein the pressure and/or volume generated by vaporizing a working fluid moves the working fluid through a heating system from a heat collector to a lower heat sink and back without external pumps, controls, or nonthermal energy input. For example, see U.S. Pat. Nos. 4,224,925; 4,241,784; 4,246,890; 4,270,521; and 4,467,862. Although useful in some situations, these passive systems present distinct disadvantages and drawbacks, being mechanically complex and subject to failure due to mineral deposition or corrosion.